The present invention relates to the field of the detection of biological aerosols.
Laser based systems are known for the detection of airborne particles of destructive biological particles such as harmful bacteria, individual or groups of cells, or protein particles. In a prior art method, a laser light illuminates air drawn through a duct which may bear the particles to be detected, such light having a wavelength near or at the peak of an absorption resonance in trace gases or chemical elements to be detected. A photodetector responds to the resulting radiation of fluorescent light due to the laser induced biochemical fluorescence. See for example, FIG. 1, col. 4-5, of U.S. Pat. No. 4,651,010 to Javan. As mentioned in this patent, it is also known to pulse the illuminating laser beam and gate the receiver coupled to the photodetector to cause it to respond in a delayed manner during a short period following each laser illumination pulse. The delay is fashioned to take advantage of the fluorescence decay time of the agent to be detected, so as to discriminate against false ambient illumination.
It is desired to improve the prior art method mentioned in the Javan patent, of laser illuminating air carrying the particles to be detected, which has been drawn through a duct, to detect the aforesaid fluorescence produced by the illuminating laser beam.